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Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental pollutants that are formed in combustion processes. At the cellular level, exposure to PAHs causes oxidative stress and/or some of the congeners bind to DNA. We investigated DNA damage biomarker levels in adults in association with indoor exposure to different PAH congeners in a panel study over two seasons. Levels of 16 EPA-PAHs were measured in house dust (collected during 3 weeks), and in the indoor air gas and particle phase (collected 24 hours before blood/urine collection). Sampling was done in 25 dwellings in Flanders, in the winter as well as the summer season. Blood and urine of the 48 non-smoker inhabitants (median age 39y) was collected during those periods. In whole blood and isolated mononucleated white blood cells (MWBC), the alkaline comet assay was performed (with and without FPG enzyme treatment). Furthermore 8-oxo2’deoxyguanosine was analyzed in urine. To assess the relationship between PAHs and DNA damage, mixed model regression analysis was done with correction for outdoor temperature, age, sex, consumption of grilled food, urinary cotinine, plasma vitamin A, and blood cholesterol levels. Naphthalene - a possible carcinogenic (IARC group 2B) compound - was in both seasons relatively the most important PAH in indoor air (70-80%). More heavy carcinogenic PAHs (c-PAHs) were – mainly in summer – hard to detect. In house dust mainly the 3 and 4-ring phenanthrene, fluorene, pyrene and the 4- and 5-ring structures chrysene and benzo[b]fluoranthene made the most important contribution to the EPA-PAH mixture. All PAH fractions in house dust were highest in winter time. DNA breaks in MWBC - but not of the whole white blood cells fraction - of the inhabitants were strongly associated with air as wells as house dust v-PAHs and c-PAHs, i.e. between 13 to 25% increase in number of breaks for doubling of the indoor PAH concentration. The indoor air v-PAHs were also associated with oxidative damage measured in whole blood and urine, respectively via the FPG comet assay and 8-oxodG. In the same way, also oxidized sites in whole blood, but not in MWBC correlated with NO2, PM10 and O3 concentrations 7 days before blood collection. Aside from lymphocytes and monocytes, which are present in the MWBC - and have a half-life of weeks to months - whole blood also contains a considerable amount of granulocytes, which have a rather short half-life of some days. In conclusion: DNA damage in isolated MWBC and the whole white blood cell fraction reflected the inhabitants’ exposure to indoor PAHs, although with different sensitivity.